Abstract

Polyvinyl alcohol (PVA) is the main pollutant in printing and dyeing wastewaters. This pollutant exhibits great demand, poor biodegradability and refractory degradation. In this study, PVA wastewater treatment experiments were conducted in a stably operating baffled anaerobic bioreactor (ABR) by using simulated PVA wastewater. The PVA degradation pathway and mechanism of the mixed dominant PVA-degrading bacterial strains were identified through the analysis of their degradation products. From the results, we inferred that PVA was degraded in a stepwise process under the synergistic action of different extracellular and intracellular enzymes produced by the mixed dominant PVA-degrading bacterial strains. In this process, PVA was first degraded into ketones, fatty acids and alcohols. It was then regenerated into acetic acid, hydrogen and carbon dioxide. Finally, these substances could be further utilized by methanogens. PVA was thus degraded completely. This study may serve as a reference for future works on the degradation of PVA in the ecological environment. It may also guide the sustainable development of PVA.

INTRODUCTION

The printing and dyeing industries have become major industrial wastewater emitters with their rapid development. Printing and dyeing wastewaters mainly contain suspended solids, grease from fibers, and various kinds of slurries, dyes, surfactants, and other substances. Printing and dyeing wastewaters are difficult to treat because of their massive discharge volumes, high chromaticity, and poor biodegradability (Barr & Aust 1994; Banat et al. 1996; Roberts 1996). Domestic and foreign works on printing and dyeing wastewater treatment have mainly focused on the degradation of slurries and dyes. Polyvinyl alcohol (PVA) is the most representative slurry present in printing and dyeing wastewaters (Liu et al. 2018).

PVA has been extensively used in the textile, printing, and dyeing industries since the 1940s because of its good abrasive resistance and durability (Liu et al. 2018). It is an ideal sizing agent for cotton fabrics (Chiellini et al. 1999; Chou 2010; Solaro et al. 2010; Zhang et al. 2013; Lin & Lee 2015). Its molecular formula is (C2H4O)n, and its structural formula is as follows:

PVA is a water-soluble macromolecular organic compound that is synthesized from monomeric vinyl alcohol (Guo et al. 2010). It is difficult to degrade under natural conditions. PVA causes extensive foaming in water because of its large active surface area. Foaming consequently results in oxygen enrichment in water that affects the respiratory activities of aquatic organisms and destroys the ecological balance of water bodies (Zhang et al. 2005). At the same time, PVA can increase the chemical oxygen demand (COD) and the viscosity of water, resulting in the pollution of receiving water bodies (Chen et al. 2013; Ye et al. 2017). Studies have shown that the COD of PVA slurry accounts for a considerable proportion of the total COD of printing and dyeing wastewaters (Chen et al. 2013).

Therefore, the treatment of PVA wastewater is crucial for the protection of the environment. The biological treatment method has become the first choice for the treatment of printing and dyeing wastewaters because it has higher degradation efficiency, lower cost and simpler operation processes (Chen et al. 2013) than physical and chemical treatment methods. In addition, it does not generate secondary pollution. The critical step in biological treatment is the screening of dominant degrading bacteria. At present, numerous single or symbiotic strains of dominant PVA-degrading bacterial species have been screened and isolated from the environment. Guo et al. (2013) screened out a single strain of PVA-degrading Bacillus amyloliquefaciens from soil samples. Kim et al. (2003) isolated Sphingomonas sp. SA3 and SA2, a pair of symbiotic PVA-degrading bacteria, from more than 800 samples of printing and dyeing wastewaters. The PVA degradation potential of these bacteria reached 95%. Vaclavkova et al. (2007) isolated a pair of the symbiotic PVA-degrading bacteria Shingomansa sp. OT3 and Rhodocococcus erythropolis from a wastewater treatment plant.

Nevertheless, the biological treatment of PVA features several drawbacks, such as the long culture periods and unstable degradative enzyme activities of PVA-degrading bacterial strains (Guo et al. 2010). In addition to the screening and isolation of dominant PVA-degrading bacterial strains, a full-scale study on the degradation mechanism of PVA-degrading bacteria and the degradation pathway of PVA must be conducted because vast amounts of PVA are discharged in actual production. The possible degradation pathway of PVA and the mechanism of PVA-degrading enzymes have been hypothesized in previous works (Shimao et al. 2000; Fujita et al. 2015; Ren et al. 2016; Song et al. 2016) but have not been studied in depth. At present, there is a consensus on the aerobic degradation pathway: PVA oxidase (secondary alcohol oxidase) oxidizes PVA to ketones in the presence of oxygen, or PVA dehydrogenase oxidizes PVA to ketones in the presence of pyrroloquinoline quinone. PVA hydroxyl groups are oxidized to ketones catalyzed by PVA oxidase (secondary alcohol oxidase) and then cleaved by PVA hydrolase. Although some studies have speculated on the possible degradation pathways of PVA, they are under different conditions, such as in the presence of oxygen. In anaerobic environments something completely different is expected to occur. In this study, the possible degradation pathway and mechanism of PVA were explored under anaerobic conditions.

In this study, PVA wastewater treatment experiments were conducted in a stably operating baffled anaerobic bioreactor (ABR) by using simulated PVA wastewater. The mixed dominant PVA-degrading bacterial strains in each chamber were screened out on media and cultured under optimized growth conditions. The PVA degradation products generated by mixed dominant bacterial strains in each chamber were detected through gas chromatography/mass spectrometry (GC/MS). The PVA degradation pathway and mechanism of the mixed dominant PVA-degrading bacterial strains were identified through the analysis of their degradation products. The results of this work could provide valuable references for research on the biological treatment and practical application of PVA.

MATERIALS AND METHODS

Materials and equipment

PVA, glucose, KH2PO4, NH4Cl, NaHCO3, and other biological and chemical reagents were purchased from Nanjing Shengjianquan Chemical Glass Instrument Co., Ltd (Nanjing, China). The sludge inoculant for the ABR reactor was prepared from anaerobic sludge collected from an anaerobic hydrolysis tank at Nanjing Budweiser Brewery Company (Nanjing, China).

Redox potential (ORP) was quantified with a 6350 ORP meter (Shanghai Renshi Electronics Co., Ltd). pH was measured by using a pHS-3C pH meter (Shanghai Precision Scientific Instruments Co., Ltd). Absorbance was determined using a UV-5500 ultraviolet spectrophotometer (Shanghai Meta-analysis Instruments Co., Ltd). GC/MS spectra were acquired with a GC–TOF Meteorological Chromatography-Mass Spectrometer (Micromass Co., UK).

Test water samples and inoculation sludge

Simulated PVA wastewater (PVA = 100 mg/L) was modified with glucose (600 mg/L), NH4Cl (50 mg/L), KH2PO4 (12 mg/L), and NaHCO3 (550 mg/L) to prevent acidification and ensure that nutrient elements, such as C and P, were available at the amounts required by microorganisms.

The water quality indexes of the simulated PVA wastewater were as follows: CODCr = 760–830 mg/L, BOD5 = 130–200 mg/L, BOD5/CODCr = 0.15–0.30, chromaticity = 80–100 times, and pH = 7–9. The initial indexes of the inoculation sludge were as follows: SV = 83%, MLSS = 28.98 g/L, MLVSS = 19.14 g/L, and MLVSS/MLSS = 0.66.

Media

  • (1)

    LB enrichment medium was prepared with 5 g of yeast extract, 10 g of tryptone, 10 g of NaCl, and 1 L of sterile water. The pH of the medium was adjusted to 7.4–7.6. The medium was then subjected to high-pressure steam sterilization for 20 min.

  • (2)

    The PVA screening medium were prepared with 1 g of PVA, 1 g of NH4Cl, 0.2 g of KH2PO4, 1.6 g of K2HPO4, 0.05 g of MgSO4 · 7H2O, 0.02 g of FeSO4 · 7H2O, 0.05 g of CaCl2, 0.02 g of NaCl, and 1 L of distilled water. The pH of the medium was adjusted to 7.2. The medium was then subjected to high-pressure steam sterilization for 20 min.

Experimental device

The ABR used for the treatment of simulated PVA wastewater in this experiment is shown in Figure 1. The reactor was made out of Plexiglas and had dimensions of 456 mm (length) × 156 mm (width) × 353 mm (height). It had six chambers (Figure 1). The first five chambers were used for wastewater treatment. The effective volume per chamber was 3.15 L. The chambers were divided into the downward flow and the upward flow sections. The bottoms of the chambers had partitions. The influent water first flowed through the downward flow chamber into the upper flow chamber and finally overflowed into the next chamber. The precipitation of sludge in the sixth chamber reduced the sludge content of the effluent. The top of the device was sealed with plastic wrap instead of the top cover to make the reactor run under anoxic conditions. The removal of organic matter from wastewater in the reactor mainly depended on sludge adsorption and microbial degradation in the sludge. Because baffles formed separate chambers in the reactor, each chamber can be used to cultivate the appropriate microbial communities in accordance with the different substrates added into the reactor. This cultivation approach enables the screening of different mixed dominant PVA-degrading bacterial strains from each chamber.

Figure 1

Treatment scheme.

Figure 1

Treatment scheme.

ABR operation

The ABR reactor was operated in continuous water intake mode. The hydraulic retention time (HRT) of the reactor after start-up was 45 h. In order to make it run steadily, the reactor was operated for about 20 days. After 20 days, influent velocity was adjusted to change the HRT, and HRT was set to 24 h. The biodegradability of the effluent of the ABR reactor was greatly improved at this HRT. Previous experiments have shown that 24 h is the optimal HRT for the treatment of printing and dyeing wastewaters in ABR reactors. The ABR reactor operated stably under the optimal HRT conditions.

Determination of the optimal growth conditions of mixed bacterial strains

A 30 g sample of sludge was collected from the first chamber of the ABR reactor (Figure 1) and then activated and enriched in 300 mL of LB liquid medium for 2 days. Then, 10% of the enrichment culture was inoculated in liquid medium with PVA as a C source for two rounds of screening and cultivation to obtain mixed dominant PVA-degrading bacterial strains. The strains were cultivated under the following conditions: pH = 4, 6, 8, or 9, PVA substrate concentration = 1 g/L, inoculum size = 5%; pH = 7, PVA substrate concentration = 0.1, 0.5, 1.0, 1.5, or 2.0 mg/L, inoculum size = 5%; pH = 7, PVA substrate concentration = 1 g/L, inoculum size = 1%, 3%, 5%, 10%, or 15%. The OD600 and the PVA content of the medium were determined after 7 days of shaking cultivation, and the optimal growth conditions were identified. The OD600 test was conducted with distilled water as a blank. Liquid medium containing only the C source without bacteria was used as a control group.

Degradation characteristics of PVA and analysis of the degradation products of PVA in each chamber

A 5 g sample of sludge from the first to the fifth chambers of the ABR reactor (Figure 1) was activated and enriched in 50 mL of LB liquid medium for 2 days. Next, 5% of the enrichment culture was inoculated into liquid medium with PVA as the C source. Two rounds of screening and cultivation were performed to obtain the mixed dominant PVA-degrading bacterial strains from each chamber. The mixed dominant PVA-degrading bacterial strains were inoculated into liquid medium under the optimal growth conditions. The ORP values of mixed dominant PVA-degrading bacterial strains from chambers 1, 2, 3, 4, and 5 (Figure 1) were adjusted to 0, −50, −50, −100, and −100 mV, respectively, on the basis of previous experimental results by using 1 g/L C6N6FeK3 as an oxidant and 1 g/L dithiothreitol as a reductant. After the adjustment, media were cultured with shaking for 7 days. The PVA concentration of the liquid culture medium was measured daily. After 7 days, the PVA degradation products generated by the mixed dominant PVA-degrading bacterial strains in each chamber were detected via GC/MS. The degradation pathway and mechanism of the dominant mixed bacterial strains were identified by analyzing the PVA degradation products generated by different mixed bacterial strains.

Analytical methods

Bacterial and PVA concentrations were quantified via the spectrophotometric method. The absorbance of the bacterial solution at 600 nm was measured spectrophotometrically. A total of 5 mL of water sample, 5 mL of boric acid solution, and 1 mL of I–KI solution were transferred to a 50 mL colorimetric tube. The solution was diluted to a total volume of 50 mL and protected from light for 20 min. Then, the absorbance of PVA solution at 690 nm was measured spectrophotometrically.

PVA degradation products were determined via GC/MS. For the sample pretreatment, 30 mL of water was added to a pear-shaped separating funnel and extracted with 3 mL of dichloromethane. The mixture was shaken for 5 min to promote emulsification and allowed to stand for several minutes. Approximately 3 mL of the lower phase of the emulsion was transferred to a centrifuge tube. Then, 3 mL of dichloromethane was added to the mixture, and the procedure was repeated. Approximately 9 mL of the emulsion was obtained after the mixture was extracted thrice with dichloromethane. The emulsion in the centrifuge tube was centrifuged for demulsification (3,500 r/min, 10 min). Thereafter, the liquid in the pipe was placed in a separating funnel. The transparent liquid obtained from the lower layer was transferred to a rotary evaporator and concentrated to a volume of approximately 1 mL at 45 °C, extracted through dehydration with anhydrous sodium sulfate, and sparged with N. GC/MS analysis was conducted with an HP-5MS quartz capillary column (30 m × 0.32 mm × 0.25 μm) without diversion by using high-purity He as the carrier gas. The sample volume was 1 μL. The flow rate was 1 μL/min, and the vaporization temperature was 280 °C. The temperature ramping procedure of the liquid sample was as follows: the temperature was initially held at 60 °C for 4 min, then increased to 250 °C at 5 °C/ min, and finally increased to 300 °C at 10/min. The temperature was maintained at 300 °C for 2 min. The temperatures of the ion source and transmission line were 240 °C and 300 °C, respectively.

RESULTS AND DISCUSSION

Determination of the optimal pH for the growth of mixed bacterial strains

After the mixed dominant PVA-degrading bacterial strains were screened out, the pH values of the microbial growth media were adjusted to 4, 6, 7, 8, or 9. The concentration trend of PVA dominant degrading bacteria at different pH values was obtained, as shown in Figure 2.

Figure 2

The concentration trend of mixed dominant PVA-degrading bacterial strains at different pH values.

Figure 2

The concentration trend of mixed dominant PVA-degrading bacterial strains at different pH values.

Figure 2 shows that when the culture medium pH changed from acidic to alkaline, the concentration of the mixed dominant PVA-degrading bacterial strains and the percentage of PVA degraded firstly increased and then decreased. The maximum bacterial concentration was observed at pH = 7.0. At this pH, OD600 exceeded 0.20, and the percentage of PVA degraded reached the highest value of more than 90%. This finding shows that acidic or alkaline conditions were not conducive for the growth of mixed bacterial strains and the degradation of PVA. The degradation ability of mixed bacteria decreased because acidic or alkaline conditions can reduce enzyme activity. Thus, the optimal pH for the growth of mixed dominant PVA-degrading bacterial strains was 7.0.

Determination of the optimal substrate concentration for the growth of mixed bacterial strains

After the mixed dominant PVA-degrading bacterial strains were screened out, the concentrations of PVA substrate were adjusted to 0.1, 0.5, 1.0, 1.5, or 2.0 mg/L. The concentration trend of PVA dominant degrading bacteria at different substrate concentrations was obtained, as shown in Figure 3.

Figure 3

The concentration trend of mixed dominant PVA-degrading bacterial strains at different substrate concentrations.

Figure 3

The concentration trend of mixed dominant PVA-degrading bacterial strains at different substrate concentrations.

Figure 3 shows that the concentration of the mixed dominant PVA-degrading bacterial strains and the percentage of PVA degraded firstly increased and then decreased as substrate concentration increased from 0.1 g/L to 2.0 g/L. The maximum concentration of the mixed dominant PVA-degrading bacterial strains was observed under the substrate concentration of 1.0 g/L. OD600 and the percentage of PVA degraded exceeded 0.20 and 94%, respectively. These results show that low or high substrate concentrations were not conducive for the growth of mixed bacteria and the degradation of PVA. An excessively low or high substrate concentration will result in the undernutrition or overnutrition of bacteria, respectively. These effects will affect the growth efficiency and PVA degradation rate of the mixed bacterial strains. The optimum substrate concentration for the growth of mixed dominant PVA-degrading bacterial strains was identified as 1.0 g/L.

Determination of the optimal inoculum size for the growth of mixed bacterial strains

After the mixed dominant PVA-degrading bacterial strains were screened out, inoculum sizes were adjusted to 1%, 3%, 5%, 10%, or 15%. The concentration trend of PVA dominant degrading bacteria under different inoculum sizes was obtained, as shown in Figure 4.

Figure 4

The concentration trend of mixed dominant PVA-degrading bacterial strains at different inoculum sizes.

Figure 4

The concentration trend of mixed dominant PVA-degrading bacterial strains at different inoculum sizes.

Figure 4 shows that when inoculum size was increased from 1% to 15%, the concentration of the mixed dominant PVA-degrading bacterial strains and the percentage of PVA degraded firstly increased and then decreased. The maximum concentration of the mixed dominant PVA-degrading bacterial strains was obtained with the inoculum size of 5%. OD600 and the percentage of PVA degraded exceeded 0.22 and 93%, respectively. This finding indicates that an inoculum size of less than or more than 5% exerted adverse effects on the growth of mixed bacteria and the degradation of PVA. An excessively small inoculum size was inconducive for the growth of mixed bacteria and the synthesis of enzymes, whereas an extremely large inoculum size will result in excessive nutrient consumption during the early growth stages of the mixed bacterial strains. The depletion of the nutrients required for the large-scale synthesis of enzymes during the late growth stage will reduce the growth efficiency of the mixed bacteria and the degradation rate of PVA. The optimal inoculum size for the growth of mixed dominant PVA-degrading bacterial strains was 5%.

The optimal growth conditions of PVA-degrading mixed bacterial strains in one chamber of the ABR reactor were as follows: pH = 7, substrate concentration = 1 g/L, and inoculum size = 5%.

Analysis of PVA degradation characteristics in each chamber

The mixed dominant PVA-degrading bacterial strains in the first to fifth chambers (Figure 1) were screened and inoculated into liquid media. The mixed dominant PVA-degrading bacterial strains of each chamber were cultured under the optimal growth conditions. Since ORP is closely related to microbial metabolic activities, the ORP values of the mixed dominant PVA-degrading bacterial strains from chambers 1, 2, 3, 4, and 5 (Figure 1) were adjusted to 0, −50, −50, −100, and −100 mV, respectively, by using 1 g/L C6N6FeK3 as an oxidant and 1 g/L dithiothreitol as a reductant. After the adjustment, the strains were cultured in a shaking incubator for 7 days. The PVA concentration in the liquid medium was measured daily. The degradation characteristics of PVA in each chamber were characterized and are shown in Figure 5.

Figure 5

Characteristics of PVA degradation in each chamber.

Figure 5

Characteristics of PVA degradation in each chamber.

Since different substrates entered each chamber in the ABR reactor, the mixed dominant PVA-degrading bacterial strains selected from each chamber also differed. Figure 1 shows that the percentage of PVA degraded gradually decreased from the first to the fifth chambers. The percentage of PVA degraded of the mixed dominant PVA-degrading bacterial strains isolated from the first chamber (Figure 1) reached 94.16% and was higher than that of bacterial strains isolated from the other chambers. The water sample entering the first chamber had a high initial COD value and can thus provide a sufficient C source for microorganisms in the chamber. This phenomenon resulted in high sludge loads, and microbial number and activity in the first chamber were higher than that in other chambers. The percentages of PVA degraded of the mixed dominant PVA-degrading bacterial strains screened out from the sludge taken from the first chamber was also higher than that of mixed dominant PVA-degrading bacterial strains screened out from the sludge taken from other chambers.

Analysis of the PVA degradation pathway and the degradation mechanism of bacteria

PVA degradation solution was extracted with dichloromethane and analyzed by using GC/MS. Compounds with high matching ratios were identified as possible degradation products. The GC/MS measurement results for the first to the fifth chambers and the degradation products were obtained and are shown in Figure S1(a)–(e) and Tables S1–S5 (Supplementary material, available with the online version of this paper).

The charts and tables show that the PVA degradation products included various organic species with complex structures. Although the products presented in each table differed because of the presence of different degrading bacteria in each chamber, the types of products were almost the same. Most of them were ketones, fatty acids and alcohols, and a few were saturated aliphatic hydrocarbons and esters. These findings indicated that the number of C atoms decreased with the prolongation of reaction time. The long-chain molecules of PVA were degraded into short-chain molecules, and the large-molecule refractory substances transformed into small-molecule biodegradable substances.

PVA is a macromolecule and a polymer organic compound with a high molecular weight. Its main molecular structure comprises C atoms that are alternately spaced with hydroxyl groups and 1,3-diol bonds. Relevant literature (Mansfield & Meder 2003) indicates that PVA degradation may be attributed to the synergistic effect of different degrading enzymes produced by the mixed dominant PVA-degrading bacterial strains. First, mixed PVA-degrading bacteria produce and utilize PVA-degrading enzymes to dehydrogenate PVA to form C–C double bonds. Small-molecule compounds, such as ketones, are produced upon the breakage of macromolecular chains. When the degree of polymerization decreases, extracellular enzymes produced by bacteria continue to cleave small molecules. At the same time, fatty acids and alcohols are produced under the action of hydrolysis and acidification. Intracellular degrading enzymes which are produced by other degrading bacteria and hydrogen-producing acetogen can continue to degrade small-molecule substances into acetic acid, hydrogen and carbon dioxide. Then these substances can be further utilized by methanogens. PVA is thus degraded completely. For example, Sphingomonas can degrade substances by producing endonucleases (Kawai 1999). Additionally, some saturated aliphatic hydrocarbons and esters may be impurity components in the solution. The degradation pathway of PVA is illustrated as follows:

CONCLUSIONS

This work investigated the process of PVA degradation by different dominant degrading bacterial strains in a stably operating ABR reactor. The mixed dominant PVA-degrading bacterial strains in each chamber were screened and cultured under the optimal growth conditions of pH = 7, inoculum size = 5%, and substrate concentration = 1 g/L. The analysis and determination of PVA degradation products in each chamber through GC/MS revealed that PVA was degraded stepwise under the synergistic action of different extracellular and intracellular enzymes produced by the mixed dominant degrading bacterial strains. PVA was first degraded into ketones, fatty acids and alcohols. It was then regenerated into acetic acid, hydrogen and carbon dioxide. Finally, these substances could be further utilized by methanogens. PVA was thus degraded completely. This study may serve as a reference for future works on the degradation of PVA in the ecological environment. It may also guide the sustainable development of PVA.

ACKNOWLEDGEMENT

This work was supported by the Primary Research and Development Plan of Jiangsu Province (No. BE2016703), the Natural Science Youth Fund of Jiangsu Province (No. BK20171017), the National Natural Science Youth Fund of China (No. 51707093), and the Science and Technology Program of the Ministry of Housing and Urban–Rural Development of China (2014-K7-010).

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Supplementary data